Try:
$(function() {
$("#search_form").submit(function(event) {
event.preventDefault();
var formdata = $(this).serialize();
alert(formdata);
$.ajax({
type: "POST",
url: "gallery.php",
data: formdata,
success: function(){alert('success');}
});
});
});
Everything works fine: http://jsfiddle.net/J6e4G/